Sick albino Cory?

[Leukippes]

I'm sure you've seen this same situation a million times, but I need to ask because I've been doing some research and all I can find are two different versions but no explanation.

I used to have an adult male betta and an albino Cory in a 5 gal tank, they get along perfectly, they're always hanging out together.
I did a full water change about 4 days ago and since then I started noticing my Cory was acting strange, found him sleeping on his side a couple times, thought he was dead, until I tried to get him and he would swim like a crazy fish.
He's usually chilling like that until I feed them and he goes from dead to crazy over food in a second.
Last night I found him sleeping upside down, vertically, head down on a plant and my husband and I were 100% sure he was dead, my husband tried to net him and he wouldn't move, it took him about 5 seconds to swim away.
I started googling and some say the fish usually die within the first 48hs of behaving like that, and some other say it's a typical Cory behavior, they like to play dead and sleep weirdly.
I bought a 10 gal tank, and two other Cory since I've been told they're better in groups and started feeding him peas. Sometimes I see him swimming normally, but most of the times he's just sleeping at the bottom, I caught him sleeping on his back once, then just now I caught him sleeping under the filter, head down, vertically.

What is going on? I doubt this is normal, I've never seen him like this before. I mean, I usually see him sleeping at the bottom so I haven't really paid much attention until i saw him sleeping on his head.

Anything I can do? It's been like this for 4 or 5 days, I didn't really see any of this before That full water change because I usually wake up in the morning, feed them and go on my way, but I do pay especial attention after a full water change.

Any ideas of what could be going wrong? Can I save him? Is he going to die or is he just being silly?

I'm attaching a photo of how I found him 15 mins ago, then he laid on his belly, started swimming again and now he's back at sleeping vertically, head down

Edit: sorry! Photo is on its side

 

[IllusionX]

Cory Doras need a school of 5-6.
Why did you do a full water change?

It probably got scared/stressed so he's acting up and plays dead. I've seen this behaviour at a friend's 5gal. It died a week later. It was also a loner in the tank..

Sent from my A0001 using Tapatalk

 

[Leukippes]

Cory Doras need a school of 5-6.
Why did you do a full water change?

It probably got scared/stressed so he's acting up and plays dead. I've seen this behaviour at a friend's 5gal. It died a week later. It was also a loner in the tank..

Sent from my A0001 using Tapatalk

We came back from vacation and the gel feeder that wasn't suppose to cloud the water did a terrible job at it. It was just filthy all over.
I've been meaning to get two more but I'm not sure they'd be comfortable in a 10gal? This is my first time owning Corys.
I've had him for 6 months now, just got two more.
Is there any way I can save him from dying?

 

[Leukippes]

Ok, I've been watching him closely and it looks like he lays on his right side when he stops swimming, like there's something wrong on his left side.

 

[Leukippes]

I was using gravel in the Cory tank, changed it for sand and now he's doing fine? Wth.
He's sleeping on his belly, no more sideways or crazy vertical sleeping, he's also moving a lot more.
I'm clueless.

 

[Leukippes]

I found my Cory dead last night, looked like he was getting better but nope.